1. Field of the Invention
The present invention relates to a solid-state imaging device and a method of driving the same, for example, an MOS solid-state imaging device used for complementary metal oxide semiconductor (CMOS) sensor cameras ready for low-voltage drive and moving images and a method of driving the same.
2. Description of the Related Art
In recent years, MOS solid-state imaging devices using a CMOS sensor have become commercially practical. MOS solid-state imaging devices amplify a signal, which is detected by a photodiode, by a MOS transistor for each cell, and feature high sensitivity.
Each cell (pixel) of the solid-state imaging devices is formed of a photodiode for photoelectric conversion, a read transistor for signal reading, an amplifying transistor for signal amplification, and a reset transistor to reset a signal charge, etc. A source of the amplifying transistor is connected to a vertical signal line, and a signal output to the vertical signal line is output to a horizontal signal line through a horizontal selection transistor.
The pixel size of the solid-state imaging devices has been reduced year after year, due to increase in the number of pixels and demand for reduction in the optical size. For example, the pixel size of CMOS sensors used for digital cameras and the like in recent years is about 2 to 3 μm. Since the number of photons which can receive light is reduced in such fine pixels, the signal-to-noise ratio cannot be maintained unless noise is reduced to compensate it. When the signal-to-noise ratio cannot be maintained, the image quality in playback images degrades, and the quality of playback images degrades. Dark current noise which flows into photodiodes during charge storage is one of the main forms of noise. Reducing dark current noise is indispensable for maintaining the signal-to-noise ratio in fine pixels.
In the meantime, in operation of reading a signal from a photodiode, it is necessary to read all charges to prevent leaving of any charges in the photodiode, to suppress an afterimage phenomenon which is caused by reading charges, which were stored in the previous frame, in the following frame. Therefore, it is impossible to increase the threshold of the transfer transistor. The dopant concentration of the channel dope layer of the transfer transistor is low, and generally about 1.0×1015 cm−2 to 1.0×1017 cm−2.
When the channel dope layer has such a dopant concentration, the semiconductor substrate interface directly under the transfer transistor becomes depleted during a charge storage period of the photodiode (during the read transistor turning off). Therefore, electrons being minor carriers are generated to form a dark current, and the dark current flows into the photodiode. The value of the flowing dark current varies pixel to pixel, and thus the dark current becomes a fixed pattern noise on the playback image, and the signal-to-noise ratio in the playback image degrades.
To deal with the above problem, a patent document (Jpn. Pat. Appln. KOKAI Pub. No. 2002-217397) takes the following measure. Specifically, a negative voltage is applied to the gate of the transfer transistor during a charge-storage period, and thereby a sufficient number of positive holes being major carriers are stored in the channel dope layer of the transfer transistor. Thereby, the speed of generating electrons in the interface of the semiconductor substrate remarkably decreases. Thus, the dark current flowing into the photodiode is reduced, and thereby the fixed pattern noise is reduced.
However, the patent document has the following problem. Specifically, when a negative voltage of, for example, −2 V is applied to the gate of the transfer transistor during a charge-storage period, a difference in potential of about 5.5 V occurs between a floating diffusion layer (detecting section) which is biased to, for example, 2.5 V during the charge-storage period and the gate of the transfer transistor. Most of the difference in potential is applied to an end portion on the detection section side in the gate insulating film of the transfer transistor. However, when the electric field applied to the gate insulating film exceeds 5×106 V/cm, generally the voltage-withstand reliability of the gate insulating film degrades, and the insulating property of the gate insulating film badly decreases during operation of the device. Thereby, the gate of the transfer transistor and the floating diffusion layer are short-circuited, and it is impossible to perform signal reading operation of pixels.
Since the thickness of the gate insulating film is about 50 Å in the general CMOS device manufacturing process in recent years, when the above difference in potential is applied to the gate insulating film, the electric field applied to the gate insulating film is about 9×106 V/cm at the maximum. Therefore, according to the device driving method of the above patent document, the reliability of the gate insulating film may degrade, and the device operation may become impossible.